The typical process of blow molding a plastic manufactured article involves the steps of downwardly extruding a section of a tubular parison of thermoplastic polymeric material from an extrusion head, sealing the opposite ends of the parison section to confine a volume of air to keep the parison from collapsing on itself, and then transferring the parison section to a mold, where it is clamped between mating mold halves on a blow molding machine. The mold consists of two halves which are opposingly separable along a vertical line. The mold halves define a mold chamber. Air is then injected into the parison section to expand it against the mold chamber to shape the manufactured article. The molded manufactured article is allowed to cool for a short period of time and then removed from the mold.
The resulting manufactured article is thus a hollow article, i.e., of double wall construction. Where a container such as a case is being made, typically two manufactured articles, a base and a lid, will be molded, and they are assembled together to form the case.
In instances where related non-identical articles, such as the lid and base of a case, are to be manufactured, they are made in a multichamber mold known as a "family mold", which allows two different manufactured articles to be made simultaneously from the same parison section.
The use of a multichamber family mold is desirable because it allows for simultaneous formation at a single workstation of related manufactured articles such as a lid and base of a case, which provides economies of labor and in-process storage and handling, and also simplifies control of the parison extrusion conditions. It is readily apparent that the separate sequential manufacture (or simultaneous manufacture in two separate workstations) of a certain number of lids followed by a certain number of bases in separate single chamber lid molds and base molds will require more man-hours than if the lids and bases were made at the same time at the same work station. Further, if the lids and bases are being made at the same time, quality control is simplified, since the polymeric extrusion that creates the parison section will be more consistent in its quality, temperature, thickness and length than if lids and bases are made in separate molding runs.
However, multichamber molds have disadvantages when articles of different depths are being manufactured. Different depth lids and bases are quite common, for example, in the manufacture of a camera case, or other specialized case designs where a deep base is to be assembled with a shallow lid.
The problem with manufacturing articles such as a lid and base of different surface areas in a multichamber mold is that substantially the same areas of a parison section are used to form the lid as the base. However, a deeper base will have a greater surface area than the lid. Consequently, since a similar amount of parison material is being stretched over a larger area in the base than the lid, the lid walls would be thicker than the base walls. In order for the base to meet minimum manufacturing specifications for wall thickness, the lid walls will be thicker than necessary or desirable. This is a waste of the polymeric material. When a large number of cases are being manufactured, the cost of wasted material can be substantial. In addition, an excessively thick lid is more likely to warp and/or present an unpleasant appearance. Moreover, the excess material increases the weight of the product, increasing shipping and delivery costs. Moreover, a thinner wall may often be used in a shallow lid than is necessary in a deeper base. It may in some instances be desirable to mold a lid which has thinner walls than the base to reduce material costs.
The problem of unequal depth lids and bases is somewhat ameliorated by the tendency of the parison section to sag, by which the weight and thickness of the upper end of a parison section is somewhat less than the lower end. This tendency favors molding a shallower article such as a lid in the upper zone of the multichamber mold, and a deeper article such as a base at the lower zone of a multichamber mold. However, parison sag is usually not sufficient to provide enough variation in weight and wall thickness between the upper and lower parts of the mold when a deep article base is combined with a shallow article.
The problem of excess wall thickness arises in the manufacture of even a single manufactured article. For example, in rectangular and other cornered articles it is necessary to design the mold and set parison extrusion conditions such that the corners have a sufficient wall thickness to provide rigidity and strength to the article. However, in blow molding a rectangular article, the corner areas will be thinner than the flat wall areas because of the mold geometry. The blow molded material is subject to stretching and thinning at the corners because the parison first contacts and adheres to the flat wall areas of the mold and subsequently stretches and thins as it expands into the corner areas. Thus, in the manufacture of such blow molded articles, the mold, parison section and process conditions are designed and selected to provide the desired corner wall thickness. When the desired corner wall thickness is obtained, the sidewalls may be thicker than necessary or desirable.